The present invention relates to the transmission of data on multiplexed channels, and more particularly to techniques for optimizing the round-trip delay experienced by data that is transmitted on multiplexed channels.
Sampled speech sources generate data traffic that typically consists of periodic packets of either constant or variable size. (The latter case comes from compression or silence removal). If a large number of speech connections are multiplexed on the same transport link, the transmission delay will include the delay caused by buffering those speech packets that arrive while the transport link is already busy transporting another packet. In the worst case, many or all packets arrive within a short time interval, consequently requiring that they be queued up for transmission. For a large number of connections this queuing delay might become the major component of end-to-end delay.
Because the speech connections send samples with a fixed inter-arrival time that is common to all of them (herein referred to as the "speech packet repetition rate"), the timing relations of packet arrivals from different connections are unchanged as long as the connections exist. This means that if a large number of connections (eventually all of them) send their packets at the same time, they will continue to do so until the connections are released. If First-In-First-Out (FIFO) buffering is used, an analysis of an idealized system would conclude that one of the connections would always have its packets arriving first, another one of the connections would always have its packets arriving second and so forth, resulting in a predictable order of buffering for each of the connections in this group. However, in practice there is some variability in each connection's exact arrival time (herein referred to as "jitter"). As a result, the order of transmission and therefore the buffering delay of packets sent in this group will depend on the jitter and will be unpredictable. Because of this unpredictability, conventional system designs need to ensure that each connection can handle the worst case delay, namely, the case when the connection's packet is the last one in the group to be transmitted.
This situation is illustrated in FIG. 1. The packets for a group of connections 101 arrive approximately at the same time with regularity, so that at time t=1, they are transmitted as a group 103 in a first order. For example, the data packet 105, supplied by the third connection, happens to have been transmitted first, and the data packet 107, supplied by the fourth connection, happens to have been transmitted fourth.
At time t=2, however, the jitter in the packet arrival times causes a different transmission order to be used. In this example, the packet 105' from the third connection is no longer first, but is instead transmitted fourth. The first packet to be transmitted (i.e., packet 109) comes instead from the first connection. A similar reshuffling of transmission order affects other connections as well.
Thus, it can be seen that when a large number of connections send their packets at the same time, the multiplexing order will cause some packets to suffer very little multiplexing delay, while others will have to wait for a long time. However, the variability in transmission order means that at any given time, any of the connections could end up having its packet transmitted last, resulting in the longest possible uni-directional multiplexing delay (referred to herein as the "delay limit"). If all connections have the same multiplexing delay limit then the dimensioning of the system must ensure that even the last packet will be transmitted in time, in other words, the performance is dimensioned for the "worst case" possibility. Those packets that are not sent last will have a multiplexing delay that is less than the delay limit, but this difference is unused, and is therefore a loss.
For voice connections, the round-trip delay is more important than the uni-directional transmission delay because significant round-trip delay noticeably affects two people's ability to speak with one another via the communications channel. Because conventional systems base their design on the assumption that each connection will experience the worst case delay, it follows that the maximal round-trip delay will contain the maximal multiplexing delay twice (i.e., once in the forward direction, and once in the return, or "backward", direction). Thus, convention systems need to be designed to accommodate a round-trip delay equal to twice the delay limit.
There is therefore a need for techniques that optimize the round-trip delay experienced by speech packets that are transmitted via a shared medium